Changes to Typeable

| Should there perhaps be a NewTypeable module which could then be renamed
| into Typeable once it is sufficiently well established?
I started with that idea, but there would be a 2-stage process:
* Step 1: (when PolyTypable becomes available) People change to import Data.PolyTypeable
* Step 2: (when PolyTypeable becomes Typeable) People change back to Data.Typeable
I was hoping to avoid that. Notice that even if we replace Typeable immediately with the new polymorphic one, doing so has very limited impact: for users who just use "deriving Typeable" and "typeOf", that will work fine with the new thing too. But I'm sure that some "seams" would show through. It's a balance.
Simon